1. Field of the Invention
The field of this invention generally relates to the mixing of gases with liquids and the separation of a dispersed phase from a continuous phase. More specifically, it relates to the aeration of liquids and separation of a dispersed phase, e.g. solid particles from a mixture comprising the dispersed phase and the continuous phase by the passage of gas bubbles through an apparatus which is vertically submerged in the liquid and within which apparatus the rising gas bubbles are broken up by mechanical action into a multitude of much smaller gas bubbles which are dispersed within the liquid.
2. Description of the Prior Art
Aeration of liquids is commonly performed, for example, to accelerate bacteriological decomposition of liquid waste, to prevent algae formation on the surfaces of stagnant pools or ponds, and so forth. The term "aeration" as employed herein is to be understood as denoting the introduction of any type of gas into any type of liquid.
The simplest method of aeration comprises introducing a gas into a liquid through holes in an appropriate supply line. Some of this gas is absorbed as the gas bubbles rise through the liquid. Unabsorbed gas escapes from the surface of the liquid, and may or may not be captured for recirculation.
In spite of its simplicity, this method is very inefficient. The gas bubbles, even if small, when introduced into the liquid, tend to aggregate into large bubbles or slugs of gas as they rise. These gas slugs have comparatively small surface-area-to-volume ratios, thus relatively little gas-to-liquid contact. This results in relatively low rates of gas absorption by the liquid at the liquid-gas interfaces. If the openings in the gas outlet are made very small to introduce small gas bubbles, fouling or plugging of the openings often occurs. In addition, the transmit time of the gas through the liquid may be quite short if the liquid container, for example, a pond or holding tank, is shallow. This short gas-to-liquid contact time further results in an inefficient rate of gas absorption by the liquid. In addition, minimum turbulence is created for disrupting the liquid-gas interfaces, disruption and renewal of the interfaces being essential for high rates of gas absorption or mass transfer.
Some slight improvement in absorption efficiency is obtained by the use of nozzles at the gas injection openings which introduce the gas into liquid in a swirling manner so as to create some degree of turbulence. This tends to delay somewhat the formation of large gas slugs and to disperse the gas bubbles through a large volume of liquid. However, high absorption efficiencies are still not obtained.
More commonly used processes employ the pneumatic (or air) lift pump principle. When a gas is bubbled up through an elongated tube which is vertically submerged in a liquid, the buoyancy force of the rising gas bubbles causes an upward lifting or flow of the liquid through the tube. This upward flow causes a circulation within the entire body of liquid, with the liquid being continually drawn into the bottom of the tube and being discharged from the top thereof. Turbulence in the liquid above the top of the tube (which is normally submerged well below the surface of the liquid) tends to improve the absorption rate of the gas by breaking up, to some extent, large gas slugs and by disrupting and renewing the liquid-gas interfaces (see for example U.S. Pat. No. 3,032,496). The liquid circulation and turbulence caused by such pneumatic lifts may also be used to prevent formation of ice on the surface of the liquid, or to reduce the magnitude of surface waves, for example in a harbor area. The absorption efficiency obtained is still much less than desired, however, because large gas slugs tend to form and remain unbroken, and because the gas-liquid contact time is not appreciably increased. Therefore, a considerable amount of gas must be pumped through such pneumatic lift tubes in order that a small amount may be absorbed by the liquid. Because of the inefficiency in the absorption process, much of the energy used to pump the gas is wasted.
Helical tube dividers installed in some pneumatic lift tubes (for example U.S. Pat. Nos. 1,144,342 and 3,452,966) increase the gas-liquid contact time by providing increased path links for the gas bubbles to travel as they spiral up through the tubes. In addition, the gas and liquid exit from the tops of the tubes with a rotational motion, thereby somewhat increasing the turbulence thereabove. However, large slugs of gas still tend to form within the tubes, with still relatively poor absorption efficiency. Some helical tube dividers (for example U.S. Pat. No. 1,144,342) are provided with holes interconnecting the adjacent chambers to help prevent formation of large gas slugs. There is still a tendency to produce small gas bubbles and the gas absorption efficiency is still much less than desired. Gas which is not absorbed in the bubble transit through the liquid is either lost or must be repumped through the liquid. This requires additional gas pumping capacity and horse power.
Because of inefficiencies of present pneumatic lift tube aerators, it has been necessary to pump relatively large amounts of gas through the liquid--only a relatively small portion actually being absorbed by the liquid--and to employ a relatively large number of pneumatic lift tubes, particularly when the liquid is contained in shallow tanks or ponds and short tubes must be used. Thus, there has been considerable wastage of gas pumping power with resulting high costs involved in such complex aerator systems.
Some aerators includes a motor-driven, horizontally rotating submerged turbine. The non-enclosed turbine is generally positioned above a source of gas bubbles and is used to break up and disperse the released gas bubbles and to create turbulence in the liquid. Other aerators employ motor driven, vertically rotating, non-enclosed turbines or paddles at, or just below, the surface of a liquid. Such aerators usually rely upon the air above the surface of the liquid, some of which becomes entrapped in the churning liquid, for aeration. However, motor-driven aeration systems are expensive to produce, to operate, as well as to maintain. A source of power for the motor must also be available.
Most recently, an aerator having high efficiency for dispersing the gas in the liquid is set forth in U.S. Pat. No. 3,969,446. The aerator comprises an elongated tube having openings at both ends and having mounted herein one or more turbines which are free to rotate about the longitudinal axis thereof. The tube is vertically submerged in a liquid, for example, in a lake or pond of water. Air or another gas is supplied to the lower end of the tube. Gas bubbles rising through the tube cause an upward flow of liquid therethrough. The turbines are rotated solely by this upward flow of gas and liquid. This rotation of the turbine causes the gas bubbles to be broken up into a vast number of much smaller gas bubbles which are dispersed throughout the liquid so that optimum gas absorption may occur. When more than one turbine is used, the turbines are so constructed that adjacent turbines rotate either at different speeds or in counter-direction to thus optimize the breaking up of the gas bubbles. Although this devise provides improved aeration efficiency, it suffers from the disadvantage that, when pumping liquid waste which contains such materials as hair, the hair becomes entangled in the turbine blades, thus reducing the efficiency of the aerator.
Thus, the present invention provides a method for mixing gases with liquids which may contain solid matters, e.g. hair, which will plug or foul aeration devices known in the prior art. Furthermore, this invention provides a method for separating a dispersed phase which may be a liquid or solid, from a continuous liquid phase.